1. Field of the Invention
This invention relates to control of the dump-valve of turbocompressors.
2. Description of the Prior Art
It is known that heat engines supplied by a turbocompressor always include a dump valve bypassing the turbine side of the turbocompressor so that the supply pressure to the engine will not increase indefinitely to a dangerous level.
To control this dump valve, it is known to use a diaphragm-equipped pneumatic actuator with a return spring to close the valve, and to allow pressure collected at the outlet of the compressor portion of the turbocompressor to act in such a way as to open the actuator. This device has the advantage of being simple and allows the intake pressure of the engine to be regulated by means of a gradual release of pressure, while keeping said pressure from reaching a dangerous level.
It is also known that for low engine loads, pressure and exhaust gas output are not enough for the turbocompressor to provide a significant supply pressure. By contrast, beyond a certain threshold at which the turbocompressor starts to become effective, the supply pressure increases significantly, simultaneously increasing the filling rate and therefore the pressure and output of exhaust gas. By retroactive effect, the supply pressure continues to increase. Thus one moves starkly from the atmospheric phase into the supercharged phase, with the latter bringing on a posteriori, the intervention of the above regulating means. However, in spite of its simplicity the device mentioned above has the drawback that, in the atmospheric phase the turbine side of the turbocompressor constitutes an exhaust pressure drop which decreases the efficiency of the engine as well as cooling the exhaust gasses by increasing calorific losses, which is a disadvantage when an anti-pollution catalytic converter is used. It would therefore be preferable to eliminate both the pressure drop and the calorific loss during the atmospheric phase.
It is also known that a turbocompressed engine can, under transitory conditions, tolerate a supercharging rate much greater than that permitted under stabilized conditions. This is explained particularly by the fact that the temperature of the surfaces in contact with gasses in the engine is a priori not as high at the beginning of acceleration as under equivalent stabilized conditions and also by a better cooling capacity of the heat exchanger. Other factors, such as enrichment during acceleration, help to allow a momentary increase in the supercharging pressure limit. It would thus be preferable, in the simple device mentioned above, to delay the moment at which the dump valve comes into play during periods of acceleration.
In addition, for reasons of engine safety, the operation of the dump valve should allow the intervention solely of the pressure downstream of the butterfly valve, i.e., the actual supply pressure, which is generally lower than the pressure upstream for all positions of the butterfly valve except wide open, which also means restricting the operation of the dump valve.